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Memory

Posted on 12 May 2008 at 11:05

Your chosen mix of processor and motherboard has a big impact on the memory you can use in your system. For a start, current AMD processors can use only DDR2 memory. Ever since the Athlon 64, AMD has integrated the memory controller into the processor itself, meaning AMD needs to introduce a whole new line of CPUs to provide support for new memory types. Phenom CPUs with DDR3 support are due later in 2008, but new motherboards and chipsets will be required to take advantage of the DDR3 capability. So, for now, if you choose AMD you'll be limited exclusively to DDR2.

With an Intel Core 2, you can opt for DDR2 or DDR3, depending on your choice of motherboard and chipset, as this dictates support for memory (although there are a few boards that support both types). DDR3 is still considerably more expensive, and it remains questionable whether it's worth the extra outlay. In our benchmarks, DDR3 memory made less than 1% difference, yet it costs three times as much. However, opting for DDR3 does give you much more room to upgrade to take advantage of processors with a faster FSB. DDR2 won't get much faster, and this is why we've chosen DDR3 for all but our low-cost AMD-based system.

Be sure to get the right type of memory for your motherboard, too. Since DDR2 and DDR3 work in different ways, they're neither interchangeable nor backwards-compatible. Although the modules for DDR, DDR2 and DDR3 all have the same physical dimensions and use the same type of connection (184 pins for DDR, 240 pins for DDR2 and DDR3), they're differentiated by the position of a notch in the middle, so you shouldn't be able to fit the wrong memory. If the module you have won't go in either way round, don't try and force it in, either - this will most likely result in permanent damage to both memory and motherboard. In addition, server-orientated registered ECC memory has a notch in a different place and won't fit in a motherboard without support for this type of DIMM.

Beyond the basic difference between DDR2 and DDR3 lies a whole range of alternative memory speeds. These are denoted by their JEDEC transfer rates, which can be quoted in two different ways. A name like DDR2-400 refers to the bus speed in MHz, but not the true peak data rate. This is expressed in the form PC2-xxxx (or PC3-xxxx for DDR3 modules). The larger the number, the faster the memory. However, raw sustained throughput isn't the whole story. The speed at which the memory reacts to requests, called latency, is also important. This is denoted by four numbers in the format 7-7-7-20. Here, lower is better. In general, DDR2 offers lower bandwidth but less latency.

The combination of these two features is what differentiates cheap memory from the premium variety. The most expensive modules will offer the fastest bandwidth with the lowest latency. However, your motherboard will only support a certain maximum memory frequency. Intel's X48 chipset alone natively supports 1,600MHz DDR3 memory, and some chipsets will have an official ceiling of 1,066MHz. Faster memory is aimed at those who wish to push their memory to higher clocks manually. If you don't plan to do this, it's not worth the extra cash. In this case, the fastest memory officially supported by your chipset is the best option. So, for both our games PC and our silent system, we've chosen 1,333MHz PC3-10600 memory, in the shape of twin 1GB modules of Corsair TwinX XMS3. For our budget system, we've chosen PC2-6400, offering 6,400MB/sec of bandwidth with 5-5-5-18 timings. This runs at 800MHz, the top bus speed supported by the ASRock motherboard.